The present application relates to frame synchronization architectures and techniques, particularly in time-division-multiple-access (xe2x80x9cTDMAxe2x80x9d) mobile phone systems.
Mobile communications are a very important area of the electronics industry, and have become an important part of the lives of most citizens in developed countries. One common mobile communications architecture is cellular phone systems. In such systems a mobile phone will establish communication with a nearby stationary transceiver when it is turned on, and the stationary transceiver station provides a connection into the telephone system. The mobile phone will then switch its communication to other stationary transceivers as a user drives around, so that it is always in touch with some transceiver. Thus each transceiver defines one xe2x80x9ccellxe2x80x9d of wireless interface. Each stationary transceiver can interface to multiple mobile umts at once, and each mobile unit can transfer to a new transceiver when it gets too far from its previous transceiver.
More recently satellite telephone systems have been proposed. In such systems the mobile units can interface to communications satellites rather than to stationary ground units. In some systems low-Earth-orbit satellites are used, since these are many times closer to the surface than are geosynchronous satellites. However, low-Earth-orbit satellites move around the Earth fairly quickly, so a mobile station must be able to transfer from one satellite to another. Here too one satellite transceiver must be able to interface to many mobile units.
Thus each base transceiver must be able to talk to multiple mobile units. One of the basic techniques is TDMA, as described below. For example, TDMA techniques are used in the Digital European Cordless Telephone (xe2x80x9cDECTxe2x80x9d) system (which is used for mobile telephony in Europe), the Japanese Personal Handy-Phone System (xe2x80x9cPHSxe2x80x9d), and the MIL-STD-88-183 for UHF satellite systems.
One general problem in telecommunications is how to carry multiple signals on one communications channel. TDMA is one of the basic techniques for doing this. (Other quite different ways to do this are frequency division multiple access, or xe2x80x9cFDMA,xe2x80x9d and code division multiple access, or xe2x80x9cCDMAxe2x80x9d). In TDMA, a transmitter sends out a series of xe2x80x9cframes.xe2x80x9d A frame is a sequence of symbols which contains data for several different destinations, strung together in a definite format. Each receiver can (ideally) pick out just the data it needs, from just one part of the frame.
One requirement of TDMA is that each receiver must know where the frame starts. That is, each receiver must become synchronized to the transmitter. This can be difficult.
More precisely, a receiver must synchronize in three ways:
it must synchronize to the transmitter""s carrier wave (in an RF system);
it must synchronize to the transmitter""s symbol rate and phase, so that the receiver knows where one symbol starts and another ends; and
it must synchronize to the transmitter""s frames, so that the receiver knows where one frame starts and another ends.
Each of these synchronizations presents its own difficulties. The present application is particularly directed to the problems of frame synchronization.
FIG. 10 shows a typical TDMA transmission bit stream. The preamble is comprised of a known series of binary bits, called the bit_sync pattern. These bits permit the receiver to acquire the symbol synchronization, and are followed by a unique word which signals the start of a frame. The unique word is the frame_sync pattern (bits ui).
Note also that, although the bit_sync pattern is fixed, there is an unknown timing offset between the bit_sync pattern and the start of the unique word. Thus, no simple concatenation of the bit_sync pattern with the unique word will accurately represent the incoming signal stream in all cases.
If frame synchronization errors occur, the receiver will not be able to pick out the correct bits in the stream of incoming bits (most of which are meant for other receivers). However, frame synchronization presents some difficulties. The start of a frame is indicated by a special sequence of bits (called the xe2x80x9cunique wordxe2x80x9d), but it is still necessary for a receiver to recognize this unique word in a continuous stream of bits.
To detect the unique word when it occurs, the receiver typically uses a binary finite-impulse-response (xe2x80x9cFIRxe2x80x9d) filter structure. Conventionally this is operated as a simple correlation detector. As shown in FIG. 12, the filter coefficients are simply the bits ui of the unique word U itself, which are multiplied with the bits ri of the data stream. A binary register 100xe2x80x2 holds the bits ui of the unique word, and provides them as outputs on 1-bit lines 102xe2x80x2 to the combinatorial stages 104xe2x80x2 (which are multipliers in this example, but could also be XOR gates). A series of delay stages 108xe2x80x2 clock the input bits ri through sequentially to the combinatorial stages 104xe2x80x2. The outputs of the combinatorial stages 104xe2x80x2 are summed equally by a summer 110, to produce a raw correlation output xcex3 (which can optionally be normalized by a stage 1212, to provide a numerically accurate measure of correlation). The output will reach its highest value when each 1 in the coefficient is multiplied with a 1 in the data stream. In this architecture the receiver concludes that frame synchronization has been achieved when the correlator output crosses a threshold. Alternatively, if a bit rotation (shift) of the unique word is used instead, the correlator would operate with the same performancexe2x80x94no better and no worsexe2x80x94except that a different phase shift would be present at the time when the unique word was detected.
FIG. 3 shows a plot of the correlation values for a conventional correlation receiver for a unique word of length 24, assuming that a maximum of 8 bits precede the frame sync (Lb=8). The variation in correlations as the different phases of the correlation vector are tried can produce xe2x80x9csidelobes,xe2x80x9d i.e. spurious local maxima in the correlator output. Such a sidelobe occurs, in this example, at a shift of five units. In conventional systems, such a sidelobe can cause a false detection of frame synchronization, and this can lead to an attempt to lock onto an incorrect frame synchronization value. (The high value at shift 9 shows the correct acquisition of frame synchronization.)
In terms of the system context, a false acquisition of a sidelobe peak can mean complete loss of synchronization between the mobile unit and the system. This will require reacquisition of correct timing from scratch. This can take seconds in some systems. Thus, in a cell phone context, this may result in a noise burst, or in complete loss of a call.
The present application discloses that unique word detection (and hence frame synchronization) can be improved by using a correlation coefficient vector which is not equal to the unique word (nor to any shift or multiple of it), but which is dependent on the unique word. In some embodiments, the correlation coefficients are dependent both on the unique word and also on the bit_sync pattern. Preferably the coefficient vector is optimized by picking it to be xe2x80x9cnearxe2x80x9d a set of error offset vectors (using a special minimax metric), to maximally distinguish the unique word from the erroneous candidates. The error offset vectors are derived from a set of erroneous candidate vectors which are all of the same length as the unique word, and which are found within the concatenation of the unique word with the bit_sync pattern.
The disclosed optimal linear receiver provides better sidelobe suppression than the conventional correlation operation. The advantageous result is to decrease the potential for a false detection of the sidelobe thereby increasing the potential for detecting only the desired peak value. Another advantage is easy implementation, since the existing systems can be upgraded simply by a change in correlation coefficients in the receiver. Another advantage is that irrespective of the length of the sequence, the innovative receiver always yields gain over the conventional correlation receiver. Another advantage is that the innovative method achieves far greater gain for shorter length sequences. Another advantage is that joint optimization of the unique word and the linear receiver will achieve even better synchronization performance.